A student observes a pond and notices tadpoles swimming near the surface. Over several weeks, she records the following changes:
• Week 1–3: Tadpole has a long tail and gills, breathes underwater.
• Week 4–6: Hind legs appear, then front legs; tail begins to shrink.
• Week 7–8: Tail is fully absorbed; the animal begins to breathe air and moves onto land.
The student wonders why the frog must return to water to reproduce even though it now lives on land. Which statement BEST explains this?
AThe frog must return to water to reproduce because its eggs have no protective shell and would dry out on land, and its sperm must swim through water to reach and fertilise the eggs.
BThe frog must return to water to reproduce because adult frogs cannot breathe air for long periods and need to stay near water to survive.
CThe frog must return to water to reproduce because tadpoles need to eat water plants, so the eggs must be laid where food is available.
DThe frog must return to water to reproduce because the water provides warmth that is needed to trigger the hatching of the eggs.
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Worked Solution
Step 1: Identify what is unique about amphibian reproduction.
Amphibians like frogs are called amphibians (Greek: "double life") because they live both in water and on land. As adults, they breathe air. However, they still depend on water for reproduction.
Step 2: Analyse the egg structure.
Frog eggs (spawn) are covered only in a soft, jelly-like coating — they have no hard shell like reptile or bird eggs. Without water, this jelly coating dries out rapidly and the embryo dies. The eggs MUST remain moist.
Step 3: Consider fertilisation.
Male frogs release sperm into the water as the female lays eggs. The sperm must swim through water to reach and fertilise the eggs. External fertilisation in water is the defining feature of amphibian reproduction.
Step 4: Evaluate each option.
Option A correctly identifies BOTH reasons: eggs would desiccate without water AND sperm need water to swim to the eggs. Options B, C, and D each contain a partial truth or outright error that does not address the core reproductive constraint.
Conclusion: The frog returns to water because its eggs lack a protective shell (and would dry out on land) and because fertilisation requires water as the medium for sperm to reach the eggs.
Correct answer: The frog must return to water to reproduce because its eggs have no protective shell and would dry out on land, and its sperm must swim through water to reach and fertilise the eggs.
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